1. Field of the Invention
The present invention relates to a pneumatic tire comprising a silica-rich compound tread rubber provided with an antistatic conductive rubber layer. It particularly relates to a pneumatic tire having improved safety based on durability and extrusion productivity.
2. Description of the Related Art
In a prior pneumatic tire, a proper amount of carbon black is contained in tread rubber, and there is no problem relating to electrical resistance of a tire or a problem relating to accumulation of charge amount. However, environmental concerns have recently been serious, and a movement of lowering fuel consumption of vehicles has been accelerated. In order to attain low fuel consumption, i.e., mitigation of rolling resistance by improving tread rubber, it is necessary to decrease carbon black as a cause for producing hysteresis loss, and there is today given attention to silica --containing tread rubber by decreasing an amount of carbon black, as tread rubber having excellent low fuel consumption performance. In order to coexist running performance and low fuel consumption performance of a tire at a high level, particularly in a pneumatic radial tire having cap/base structure of a tread, there is a tendency to increase cases of using silica-rich compound rubber as rubber for a cap layer. As a result, there is newly caused a problem relating to electrical resistance and a problem relating to accumulation of a charge amount.
As a solution of such problems, for example, as disclosed in European Patent Specification No. 658 452, there has been known a technique of inserting a conductive rubber layer 20 from a tread surface to tread underlayer rubber at the width directional center portion of a tread 10 (FIG. 7). In this pneumatic tire, as illustrated, a connected face between the conductive rubber layer 20 and the silica-rich rubber layer 10 provided in the tread is perpendicular to the ground-contact surface of the tire.
As described above, a pneumatic tire in which the conductive rubber layer is provided to be perpendicular to the ground-contact surface of the tire sufficiently exhibits antistatic effect because the layer is constantly grounded, but the conductive rubber layer and the silica-rich rubber layer tend to peel off at the connected face due to strain produced in the tire when running, and thus there is a problem from the aspect of tire safety. That is, as disclosed in the above European Patent Specification No. 658 452, if the conductive rubber layer is inserted from the surface to the underlayer rubber in the silica-rich compound tread, in such a case that the tire receives the transverse directional force particularly when the vehicle receives a side wind during high speed running, as shown in FIG. 8, the conductive rubber layer 20, which is heterogeneous from the other tread portion 10, peels off from the boundary face, and this peeling-off has a problem to be continuously extended on the circumference of the tire. Such problem also occurs at the time of revolving and running in like manner. Once the peeling-off occurs, tire durability is considerably lowered.